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Geochemistry and tectono-magmatic affinity of mafic extrusive and dyke rocks from the ophiolite melange in the SW Zagorje-Mid-Transdanubian Zone (Mt. Medvednica, Croatia)

Mafic extrusive rocks occasionally intersected by dolerite dykes are the most abundant fragments of oceanic crust in the Early Callovian to Late Valanginian ophiolite mélange exposed at Mt. Medvednica located at the SW tip of the Zagorje-Mid-Transdanubian Shear Zone. The mélange archives a chaotic mixture of various extrusive rocks which may be analogues of the oceanic rocks derived from (at least) two different tectonic settings. The extrusive and dyke rocks of this mélange are geochemically classified into three magmatic groups: high-Ti group of N-MORB-like with arc signatures (Ti/V = 26.3-38.5), medium-Ti group (Ti/V = 20.2-23.0) and low-Ti group showing untypical LREE depleted [(La/Sm)cn = 0.49-0.62] IAT-like signatures (Ti/V = 13.2-18.0). All three geochemical group of magmas were successfully monitored by clinopyroxene geochemistry. The magmas have consistent initial Nd-isotopic values (ε Nd(T=163 Ma) +5.9 to +6.3) and initial 87Sr/86Sr ratios (0.703748– 0.704286) suggesting a similar parental source material related to the initial stage of an intraoceanic subduction of the back-arc basin ridge. It represents the new crust crystallized from melts derived from LREE extremely depleted MORB-type mantle which endured onset of increasing concomitant metasomatism and partial melting in a suprasubduction fore arc setting. K-Ar ages on plagioclase separates indicate Uppermost Bathonian-Lower Oxfordian magmatic activity between 165 ± 3.3 Ma and 160 ± 3.3. Geodynamic model for Mt. Medvednica ophiolites as a part of the Repno oceanic domain of Meliata-Maliak oceanic system was proposed on account of overall data. There are many common geochemical and geological correlative features between Mt. Medvednica and Szarvaskö ophiolites located NE in Hungary to integrate them in the same oceanic domain, which is not obvious in the case of Dinaridic ophiolites.

geochemistry; mafic extrusives and dykes; ophiolite mélange; back-arc basin; N-MORB – SSZ interaction; Zagorje-Mid-Transdanubian Zone; Mt. Medvednica

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